In recent years, the range of applications for magnetic recording apparatuses such as magnetic disk drives, flexible disk drives and magnetic tape drives has been markedly increased and the importance of such apparatuses has also increased. Also, the recording density of the magnetic recording media used in such apparatuses is being largely increased. In particular, a steeper increase in areal recording density followed the introduction of an MR head and a PRML technique in a hard disk drive (HDD). Since the introduction of a GMR head and a TuMR head in recent years, the recording density has been increasing at a pace of about 100% per year.
As a magnetic recording system of HDD, a so-called perpendicular magnetic recording system has recently extended its applications rapidly as a technique to replace a conventional longitudinal magnetic recording system. In the perpendicular magnetic recording system, crystal grains of a recording layer for recording information has an easy axis of magnetization in the direction perpendicular to a substrate. The easy axis of magnetization is an axis in a direction of which magnetization easily points. In the case of a conventionally used Co-based alloy, the easy axis of magnetization is an axis (c-axis) parallel to the normal line of a (002) crystal plane of a hexagonal closest-packed structure of Co. Therefore, an influence of a demagnetizing field between recording bits is small even when the recording density increases, and the magnetization is magnetostatically stable.
In ordinary cases, a perpendicular magnetic recording medium has a seed layer, an intermediate layer, a magnetic recording layer and an overcoat formed in this order on a nonmagnetic substrate. Also, in many cases, a lubricating layer is applied on the surface after film forming of the overcoat. Also, a magnetic film called a soft-magnetic under layer is ordinarily provided under the seed layer. The seed layer and the intermediate layer are formed for the purpose of further improving the characteristics of the magnetic recording layer. Specifically, they have a function of aligning crystals in the magnetic recording layer and also of controlling the shape of magnetic grains.
In order to increase the recording density of the perpendicular magnetic recording medium, it is necessary to realize low noise while maintaining thermal stability. Two methods are ordinarily used so as to reduce noise. One method is a method of decreasing the magnetic interaction between magnetic crystal grains in the intra-film plane direction by magnetically separating and isolating magnetic crystal grains of the recording layer. Another method is a method of decreasing the grain size of magnetic crystal grains. The method includes, for example, a method of adding SiO2 to the recording layer to form a perpendicular magnetic recording layer having a so-called granular structure in which magnetic crystal grains are surrounded with the grain boundary region containing a large amount of SiO2. However, when the noise is reduced by such a method, it is necessary to increase the perpendicular magnetic anisotropy (Ku) of magnetic crystal grains so as to ensure thermal stability. However, when the magnetic anisotropy energy is increased, Hc and Hc0 increase and writing at the recording head becomes insufficient, resulting in a problem such as deterioration of reproducing characteristics.
As a method for solving the problem, a so-called composite media has been proposed in which an auxiliary layer made of soft magnetic grains is provided on or under the perpendicular magnetic recording layer (main recording layer) having the above granular structure (for example, Non-Patent Document 1, Patent Document 1). Because of exchange coupling between a hard magnetic film as the main recording layer and a soft magnetic film as the auxiliary layer, it becomes possible to invert at a lower applied magnetic field than that of a conventional perpendicular magnetic recording medium if the auxiliary layer portion initiates magnetization inversion upon application of the magnetic field from the head first. It is also possible to control exchange coupling between the main recording layer and the auxiliary layer thereby designing a perpendicular magnetic recording medium having optimum characteristics by providing a nonmagnetic film between the main recording layer and the auxiliary layer.
However, it is difficult to control exchange coupling between the main recording layer and the auxiliary layer and, as a result of deviation from the range of optimum exchange coupling, the gradation of a hysteresis loop RS decreases to less than 1 by integrating the main recording layer with the auxiliary layer whose material has a low Ku value, unfavorably resulting in an insufficient improvement of heat fluctuation resistance.
Non-Patent Document 1:
    IEEE Transaction on Magnetics, Vol. 41, pp. 537Patent Document 1:    Japanese Patent Application No. 2005-172601